1. Field of the Invention
The present invention relates to a phase tracking apparatus used in a Loran-C navigation system wherein a sampling pulse for capturing a Loran-C signal is accurately synchronized in phase with the received Loran-C signal.
2. Description of the Prior Art
A Loran (Long-range navigation)-C system employs a chain of transmitting stations including one master and two or more secondaries. The master transmitting station periodically transmits groups of nine Loran pulses as denoted by M in (a) of FIG. 1. Each secondary transmitting station similarly transmits eight Loran pulses as denoted by S.sub.1 and S.sub.2 in (a) of FIG. 1. Each transmitting station generates the above-described Loran pulses at a pulse repetition rate prescribed for each chain (in the Japanese Maritime Area, 99.7 milliseconds). In addition, each secondary transmitting station generates its secondary station Loran pulses at a unique coding delay with respect to the transmissions from the master transmitting station.
Hence, in the Loran-C signal receiving system, the difference in distance to the two fixed points represented by the master and each secondary transmitting station can be derived from the time delays T.sub.1 and T.sub.2 between receipt of the secondary pulses S.sub.1 and S.sub.2 and of the master pulses M. Thus, the location of the Loran-C signal receiving system can be identified by the intersection of Loran hyperbolics specified by the distance difference values relative to the known positions of the stations.
In the Loran-C receiving system, a particular cycle of the carrier wave in each received pulse (generally, the third cycle of the carrier wave) is tracked in order to measure the reception delay time of the pulses from the secondary transmitting stations with respect to those from the master transmitting station. The carrier wave C.sub.a of the Loran-C signal has a frequency of 100 kilohertz and, hence, a period of 10 microseconds. Loran-C signal components are described in detail in U.S. patent application Ser. No. 657,662 filed on Oct. 4, 1984, now U.S. Pat. No. 4633260.
Recently, Loran-C navigation systems have been proposed for use in airplanes and automotive vehicles. In this case, the proposed operation of a Loran-C receiving apparatus which uses a digital PLL (Phase-Locked Loop) has been to track the Loran-C pulses which are subject to a noisy environment and which must be picked up by a receiver mounted on a vehicle moving at high speed. (Refer to the document entitled "Automation and Results of Repetition of Loran Receiving System" document No. SANE 81-34 published by Space Navigation Electronics Research group of Electronics and Communication Society).
FIG. 2 is a simplified block diagram of the aforementioned conventional PLL Loran-C phase tracking apparatus. The Loran-C signal received via a high-frequency amplifier 1 and a limiter 2 is converted into a binary signal with a frequency matching that of the received signal. The binary signal is then processed to average out the influences of noise by means of a random work filter (abbreviated RWF) and is inputted to a sampling pulse generator 7 via an integration loop 5 and a proportional loop 6.
In the sampling pulse generator 7, a comparison signal related to the input signal level is supplied to a binary quantized phase comparator 3. The comparison signal is used as a sampling pulse for the Loran-C signal.
The phase tracking characteristics of the above phase tracking apparatus are determined by the RWF 4. The RWF 4 comprises an UP/DOWN counter 41 and an N-reset circuit 42, as shown in FIG. 3.
The UP/DOWN counter 41 is reset previously to N by the output signal of the N-reset circuit 42. The UP count and DOWN count (increment and decrement) operations operations of the UP/DOWN counter 41 are carried out in response to the binary output u.sup.+, u.sup.- representing phase advance and phase delay from the binary quantized phase comparator 3.
If the count value reaches either 2N or 0, a corresponding output U.sup.+ or U.sup.- is generated and in response to the output U.sup.+ or U.sup.-, the count value of the UP/DOWN counter 41 is again reset to N.
FIG. 4 shows the probability of outputting the signal U.sup.+ representing the phase advance from the binary quantizing phase comparator 3 with respect to a phase difference .theta. between the input signal and the comparison signal. The solid curve represents the probability curve for a typically noisy environment, and the dotted curve shows the characteristics for a higher-quality signal. The probability would be 1.0 over the range of 0.degree..ltoreq..theta.&lt;180.degree. and the probability would be 0 over the range of -180.degree.&lt;.theta.&lt;0 under ideal, noise-free conditions, in which case the binary quantizing phase comparator 3 would detect the phase without error. However, when noise is present, the probability is adversely affected to a degree dependent on the S/N ratio. As shown in FIG. 4, the probability extrema are centered about the values .theta.=.+-.90.degree..
It should also be noted that the zero crossing point of the received carrier wave, i.e. when the signal level is 0 (zero), the output probability is 1/2, i.e., the probability of outputting either a phase advance or a phase lag signal is 1/2
This aspect of the receiver system necessitates conversion of the input and output probability characteristics as shown in FIG. 5 in order to realize the ideal characteristics achieved by noise-free signals even when the received signal is relatively noisy.
The above-described RWF 4 is a typical loop filter exhibitting less-than-ideal conversion characteristics. The actual probability conversion characteristics of the RWF 4 are shown in FIG. 6 in the case where the reset value N equals 8. The dotted line Q in FIG. 6 represents the average number of input pulses required for the count value of the UP/DOWN counter 41 to reach 2N. The value of Q is maximized, i.e., Q.sub.max =N.sup.2 when the input probability of U.sup.+ is 1/2. It should be noted that U.sup.+ represents a probability of the count value reaching 2N.
The PLL phase tracking apparatus is so constructed that the phase tracking of the sampling pulse is carried out in accordance with secular variations in the reception timing of the third cycle of the carrier wave due to movement of the Loran-C receiving apparatus after the third cycle of the carrier wave in the Loran-C pulses is detected by the third cycle detection apparatus of the Loran-C receiving system.
However, although the count limit N of the loop filter (in the above-described conventional apparatus, RWF 4) is constant, the S/N ratio of the reception signal varies over a wide range of -.infin. dB to over positive one hundred dB. In addition, when the Loran-C receiving system is installed in a vehicle or airplane which moves at a relatively high speed, the phase tracking apparatus may not be ale to follow the phase variations of the carrier wave in the Loran-C signal and of the comparison signal due to errors in the clock signal and thus may not be able to maintain sufficient phase-tracking accuracy.